Unfortunately, it says that with Feruchemy that you "Invest that metal on one day, withdraw the power on another day." So it's clear, in my opinion, that Feruchemical stores are Invested objects.
I was talking to Josh, and we came up with the idea that perhaps there are multiple kinds of Investitures: Physical, Cognitive, and Spiritual kinds. Hemalurgic spikes and Shardblades are pretty clearly Spiritual Investitures, so they can't be Pushed. Vin could just Push on the Lord Ruler's bracers after burning the mists, so I don't see a contradiction there. But a Feruchemical store could be multiple types. The ones we saw in the Mistborn trilogy were mostly Physical, and they could be Pushed upon.